Engl E670 - Page 30

Atipfromthedesigner:
87TESTJACK
VERY IMPORTANT INFO:
PLEASE NOTE:
88 <4Z;CABLESHORTCIRCUIT
89 4Z
90 6Z
You'll find that this test circuit is a very convenient utility, as it lets you check speaker,
guitar, and signal processor patch cords (only the mono wire is tested!). Due to the
higherresistanceof longer cables, youmay find thatthe 12 ZLED (92) ratherthan the
8ZLED(91)indicatesthatthecableisO.K.
Connect a speaker cabinet (or daisy-chained cabinets) to this jack for an impedance
check using a cord with ¼" plugs. For the cable test, insert one end of the cord to this
jack.
Before testing a cable, ensure that its other end is not
connectedtoapowerampoutput,becausethis candamagethetestcircuit.Thesame
applies to cabinets equipped with a thru jack. Ensure that it is not connected to a
powerampoutputbeforeyouplugintothetestcircuit.Disconnectthecordorcabinet
after you have tested it to avoid placing unnecessary strain on the internal power
circuit.
During animpedance check, you may find that severalLEDs illuminate
or flicker at the same time. Possible causes include faulty contacts between the plug
and jack. High-level noise can induce voltage in the speakers, which also affects the
test circuit. Deviations in speaker impedance values are possible because the circuit
measures a speaker's DC resistance, and this merely provides an approximationof the
actual speaker resistance. This explains why there may be a discrepancy between the
indicated and actual values. Whenyou aresure you know the impedance of acabinet
(or of several cascaded cabinets), set the impedance selector (74, 76) to this value. In
practice,theindicatedvalueshouldnotvarybymorethanoneLEDbarfromtheactual
value.Usethick,shortcordsfortheimpedancetesttominimizeflawedmeasurements
duetohighercableresistance.
ThisredLEDflashestoindicatethefollowingresults:
1. During an impedance check, this signifiesthat theimpedance islower than4 ohms,
andpossiblyashortcircuitinthecord(testitseparately!)orcabinet.
2. During a cable check, this signifies a short circuit in the cord's wires or in one of the
plugs.
ThisgreenLEDindicatesthefollowingresults:
During an impedance check, it signifies an (overall) impedance of 4 ohms.
ThisyellowLEDindicatesthefollowingresults:
During an impedance check, it signifies a relatively unusual (overall) impedance of 6
ohms.Thismaybeattributabletospeakertolerance(perhapsyou'redealingwithan8-
ohm system), faulty cables, or a poor contact between plugs and jacks (in this case, it
maybea4-ohmsystem),oraparallelsetupconsistingofthree16-ohmspeakersorone
16-ohmandone8-ohmspeaker.
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